China has interests in preserving the North Korean state, but not enough to start World War III over.

Chinese troops march during the military parade marking the 70th anniversary of the end of World War Two, in Beijing, China, September 3, 2015.Jon Woo/Reuters

China may not endorse North Korea's nuclear threats toward the US, South Korea, and Japan, or its abysmal human-rights practices, but Beijing does have a vested interest in preventing reunification on the Korean peninsula.

Still, China's proximity to North Korea means that the US would likely alert Chinese forces of an attack — whether they gave 30 minutes' or 30 days' notice, the Chinese response would likely be to preclude — not thwart — such an attack.

China sees a united Korea as a threat.

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"A united Korea is potentially very powerful, country right on China's border," with a functioning democracy, booming tech sector, and a Western bent, which represents "a problem they’d rather not deal with," according to Tack.

The US has more than 25,000 troops permanently stationed in South Korea, but no US asset has crossed the 38th parallel in decades. China would like to keep it that way.

And without North Korea, China would find itself exposed.

Reuters, US Department of Defense

For China, the North Korean state acts as a "physical buffer against US allies and forces," said Tack. If the US could base forces in North Korea, they'd be right on China's border, and thereby better situated to contain China as it continues to rise as a world power.

Tack said that China would "definitely react to and try to prevent" US action that could lead to a reunified Korea, but the idea that Chinese ground forces would flood into North Korea and fight against the West is "not particularly likely at all."

Overtly backing North Korea against the West would be political suicide for China.

North Korean leader Kim Jong Un inspects the construction site of Ryomyong Street, in this undated photo released by North Korea's Korean Central News Agency (KCNA).Reuters/KCNA

For China to come to the aide of the Kim regime — an international pariah with concentration camps and ambitions to nuke the US — just to protect a buffer state "would literally mean that China would engage in a third world war," said Tack.

So while China would certainly try to mitigate the fall of North Korea, it's extremely unlikely they'd do so with direct force against the West, like it did in the Korean War.

Any response from China would likely start with diplomacy.

In this image made from video released by KRT on Tuesday, March 7, 2017, North Korea launches four missiles in an undisclosed location North Korea. On Monday, North Korea fired four ballistic missiles in an apparent protest against ongoing U.S.-South Korean military drills that it views as an invasion rehearsal.PACOM

Currently, the US has an aircraft carrier, nuclear submarines, F-22s, and F-35s in the Pacific. Many of the US's biggest guns shipped out to the Pacific for Foal Eagle, the annual military exercise between the US and South Korea.

But according to Tack, the real deliberations on North Korea's fate aren't going on between military planners, but between Secretary of State Rex Tillerson and the Chinese diplomats he'll be meeting with.

Even after decades of failed diplomacy, there's still hope for a non-military solution.

US Secretary of State Tillerson arrives at Haneda international airport in TokyoThomson Reuters

"There's still a lot of diplomatic means to use up before the US has no other options but to go with a military option," said Tack. "But even if they decide the military option is going to be the way to go — it’s still going to be costly. It’s not something that you would take lightly."

While no side in a potential conflict would resort to using force without exhausting all diplomatic avenues, each side has a plan to move first.

According to Tack, if China thought the US was going to move against North Korea, they'd try to use force to pressure Pyongyang to negotiate, lest they be forced to deal with the consequences of a Western-imposed order in what would eventually be a reunified Korea.

"China could bring forces into North Korea to act as a tripwire," said Tack.

Soldiers of the People's Liberation Army (PLA) Marine Corps are seen in training at a military training base in Bayingol, Xinjiang Uighur Autonomous Region, China.Reuters/Stringer

"The overt presence of Chinese forces would dissuade the US from going into that territory because they would run the risk of inviting that larger conflict themselves."

For the same reason that the US stations troops in South Korea, or Poland, China may look to put some of its forces on the line to stop the US from striking.

With Chinese soldiers in Pyongyang and around North Korea's main nuclear infrastructure, the US would have to think long and hard about bombing these critical targets.

It's pretty likely that China would try to force the "infallible" ruler's hand.

North Korean leader Kim Jong Un receives a delegation of the Communist Party of China led by Liu Yunshan, left, member of the Standing Committee of the Political Bureau and member of the Secretariat of the Central Committee of the Communist Party of China, on an official goodwill visit to the DPRK on the occasion of the 70th founding anniversary of the Workers' Party of Korea (WPK), on October 9, 2015 in this photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang on October 10, 2015.REUTERS/KCNA

Chinese forces in North Korea would "be in a position to force a coup or force Kim’s hand" to disarm, said Tack.

"To make sure North Korea still exists and serves Chinese interests while it stops acting as a massive bullseye to the US," he added.

That would be an ideal result for China, and would most certainly preclude a direct US strike.

But even if China does potentially save the day, it could still be perceived as the bad guy.

President Donald Trump speaks during a news conference, Thursday, Feb. 16, 2017, in the East Room of the White House in Washington.AP

Chinese leaders wants to avoid a strong, US-aligned Korea on its borders. They want to prevent a massive refugee outflow from a crushed North Korean state. And they want to defuse the Korean peninsula's nuclear tensions — but in doing so, they'd expose an ugly truth.

If China unilaterally denuclearized North Korea to head off a US strike, this would only vindicate that claim, and raise questions as to why China allowed North Korea to develop and export dangerous technologies and commit heinous human rights abuses.

So what happens in the end?

Chinese President Xi Jinping awaits the arrival of US Secretary of State John Kerry for a meeting at the Great Hall of the People in Beijing, China, May 17, 2015. REUTERS/Saul Loeb/Pool

For China, it's "not even about saving" the approximately 25 million living under a brutal dictatorship in North Korea, but rather maintaining its buffer state, according to Tack.

China would likely seek to install an alternative government to the Kim regime but one that still opposes the West and does not cooperate with the US.

According to Tack, China needs a North Korean state that says "we oppose Western interests and we own this plot of land."